(*sigh*) How many times do I have to repeat myself? Secession per se is not unconstitutional. However, Article IV, Section III makes it clear the Congressional approval is needed for change in the status of a state. And Article I, Section 10 makes it clear that unilateral action when such actions impact the interests of other states are forbidden. So it is the unilateral aspect of the southern action which made it illegal.
Odd then, that there was no attempt to peacefully deal with the illegality of their actions prior to initiation of the war by the Federal actions at Sumter. Even more strange that the peace commissioners were denied even a meeting with the president.
Article 4, Section. 3.
Clause 1: New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.
Clause 2: The Congress shall have Power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States; and nothing in this Constitution shall be so construed as to Prejudice any Claims of the United States, or of any particular State.
Article 1, Section. 10.
Clause 1: No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
Clause 2: No State shall, without the Consent of the Congress, lay any Imposts or Duties on Imports or Exports, except what may be absolutely necessary for executing it's inspection Laws: and the net Produce of all Duties and Imposts, laid by any State on Imports or Exports, shall be for the Use of the Treasury of the United States; and all such Laws shall be subject to the Revision and Controul of the Congress.
Clause 3: No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.
Art 4, Sec 3, Cl 1: In seceding, a state neither forms a new Sate nor forms or erects a State within the Jurisdiction of any other State.
Art 4, Sec 3, Cl 2: Irrelevant to the issue of secession.
Art 1, Sec 10, Cl 1: In seceding, a state does not enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
Art 1, Sec 10, Cl 2: In seceding, a state does not lay any Imposts or Duties on Imports or Exports.
Art 1, Sec 10, Cl 3: In seceding, a state does not lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War.